Various hydrocarbonaceous feeds, including petroleum distillates and hydrocarbonaceous liquids obtained from coal, tar sands, and oil shale contain sufficient quantities of nitrogen-containing compounds to limit the performance of acid catalytic refining processes. Nitrogen-containing molecules tend to be basic and can poison or neutralize acid sites on the cracking catalysts. Neutralization of acid sites significantly reduces the catalyst's ability to convert heavier feeds to more desirable lighter liquid products, such as gasoline and diesel fuel. This problem is significantly more difficult among feeds from the West Coast of the United States, since these feeds often contain higher amounts of these nitrogen-containing compounds. It would be advantageous if there were a method to prevent or deter this neutralization in an efficient manner, without expensive additional equipment or other materials being introduced into the system. For fluid catalytic cracking processes, it would be especially advantageous for the process to be low-pressure, and non-hydrogen consuming. The present invention seeks to provide such a process.
The prior art has addressed a number of ways for dealing with the nitrogen-poisoning problem in fluid catalytic cracking processes. As discussed in "Fluid Catalytic Cracker Catalyst Design for Nitrogen Tolerance", G. W. Young, Journal of Physical Chemistry (Vol. 90, 1986), pp. 4894-4900, the work of Mills et al, Journal of American Chemical Society (Vol. 72, 1950) pp. 1554, demonstrates the ability of organic nitrogen compounds to severely affect the activity of cracking catalysts under ordinary cracking conditions. Among the compounds studied which demonstrate catalyst-poisoning effects are quinaldine, quinoline, pyridine, piperidine, decyclamine, analine acridine, carbazole, naphthylamine, dicyclohexylamine, and pyrrole. As discussed in Young, refiners have traditionally attempted to deal with high nitrogen feeds in a number of ways. These methods include: (a) hydrotreating, (b) acid treatment to remove basic nitrogen compounds, (c) injecting acid into the feed, (d) changing the process conditions, for example, increasing reaction temperature or the severity, i.e., the catalyst to oil ratio, (e) blending feedstocks to limit the concentration of nitrogen compounds, and (f) using more active catalysts.
The present invention seeks to provide an additional method to reduce the impact of nitrogen poisons in the feed. Specifically, it provides a mechanism by which a portion of the circulating inventory of the active acid catalyst of a catalytic cracking process is contacted with the feed prior to the feed entering the catalytic reaction zone. This initial contacting allows for much of the nitrogen to bind with a minority of acid sites within the entire inventory, and thereby hinder catalyst poisons from interfering with the main reactions of the process.
This process occurs at a temperature and for a time sufficient to strongly bind some or all of the reactive nitrogen contaminants to the separated portion of the catalyst. By "sacrificing" a minority of the acid sites of the entire inventory to bind with much of the nitrogen, the now nitrogen-reduced feed passes through the reactor with the majority of catalyst free to perform cracking relatively unhindered by the bound nitrogen poisons. Nitrogen can bind with acid catalyst surfaces in different ways. For example, a low-energy bond is formed primarily by physical adsorption. This bond or association can be made at low temperatures, but allows nitrogen molecules to come off the surface easily and migrate to titrate active sites, of perhaps even greater activity, elsewhere.
A stronger bond is formed by chemical adsorption, also termed chemisorption. A chemisorption bond has greater binding strength than the relatively weak physical adsorption bond, and therefore prevents bound nitrogen-containing molecules from migrating to other sites. We have found the temperature range of 450.degree. -850.degree. F. most effective for binding nitrogen-containing molecules to cracking catalysts. Experiments have also shown that the binding effect can be used to pre-concentrate nitrogen molecules in the cracking process to free other catalyst from this poison, thereby resulting in higher yields of conversion products. Higher temperatures than specified above result in cracking, leaving nitrogen in smaller molecules, which can rapidly migrate between sites. While the present process is particularly appropriate for high nitrogen feeds, i.e. those containing over 1000 ppm nitrogen, it can also be advantageously used feeds containing lower amounts.